Every fall, millions of monarch butterflies travel south to Mexico and take refuge in twelve mountain sanctuaries of oyamel fir forests. Now, declining numbers of the overwintering butterflies expose the migration’s vulnerability and raise questions about threats throughout the monarch’s lifecycle.
A study published online last spring in Insect Conservation and Diversity shows a decrease in Mexico’s overwintering monarch butterflies between 1994 and 2011. The butterflies face loss of wintering habitat in Mexico and breeding habitat in the United States. Extreme weather, like winter storms in Mexico and the ongoing drought in Texas, adds yet another challenge.

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The seeds of the decline date back more than 40 years as commercial and subsistence logging—now illegal in the Monarch Butterfly Biosphere Reserve—have fragmented the forest on which the butterflies depend for winter survival. Between 1971 and 1999, logging degraded 44 percent of the high-quality overwintering habitat within the reserve.

Thanks to its shelter and humidity,"“the forest serves as an umbrella, a blanket and a hot water bottle for the butterfly," said biologist Lincoln Brower of Sweet Briar College in an interview with mongabay.com. Brower, who has studied monarchs for 56 years, compares removing even a single tree to cutting a hole in the blanket.